Residual Pressure Valve won't fit into repro master cylinder!

[DAVEN1256]

In a previous thread, I was looking for advice on where to buy a new correct looking master cylinder for my '68 with power disc/drum brakes. Correct as in being the same shape as the original casting and having the double bleeder valves.......I did buy the new master cylinder but now there is new wrinkle. I am starting a new thread on it here rather than trying to keep it going at the end of the old thread were it might get missed.

http://www.camaros.org/forum/index.php?topic=15112.0

In that thread, the subject of RPV's (Residual Pressure Valves) came up. I had never heard of an RPV valve. This is what I learned about them. The RPV valve is a small rubber valve that sits behind the brass seat of a master cylinder port going to drum brakes. It keeps 10 psi in the line to spread the brake shoes closer brake linings to make contact quicker and prevent nose diving of the car when the brakes are applied.  A car with front and rear drum brakes should have an RPV in both master cylinder ports. A disc/drum set up would only have the RPV in the port going to the rear drums.

Another thing I found out is that reproduction master cylinders are being manufactured without any RPVs. The Chassis Service Manual says they are necessary. JohnZ confirmed this in an older thread also.

When I got my new master cylinder, I found there was no RPV behind the brass seat of the rear drum port. So I ordered a master cylinder rebuild kit for NAPA just to get one RPV Valve out it. When I pulled the brass seat out of the rear port of my new master cylinder to put the RPV in, I realized the port was machined too small in length and diameter to accept the RPV. Also too small in diameter to use the replacement brass seat from the kit......So now I am in a pickle and trying to figure what my next move will be.

So what I am looking to find out is.....has anyone bought a correct shaped master cylinder with the double bleeder valves that came without an RPV valve and successfully installed one? If so, I would like to know where you got your master cylinder.
Thanks....Dave

[janobyte]

Actually my casting # is 5468309(69, couple months in)  Accepts the rod, same depth as what I took off.

Brass from my old one appears the same diameter as those in the kit, which fits the bore in my new one. Will let you know how this plays out.

Also purchased from Ecklers, not HBC.    MC309-187432

[DAVEN1256]

I'll be anxiously waiting to find out if you were successful.

Dave

[jeff68]

If there is enough meat left, is it possible to carefully drill a little deeper to install the RPV.  Also, is it possible that there are different size RPVs (or even brass inserts) for drum/drum versus disc/drum master cylinders?  Did you get a drum/drum or disc/drum MC rebuild kit?

I had to install RPVs into my drum/drum MC because the shop that sleeved it apparently thought they weren't necessary.  It was a relatively straight-forward job:
http://www.camaros.org/forum/index.php?topic=2938.msg19259#msg19259

Here are a few more diagrams showing the RPV (check) valves:

[DAVEN1256]

Jeff, thanks for the diagrams...

To address you questions:

1) There is plenty of meat in there to drill out and I had given that some thought. You would have to drill deeper and to a larger diameter though. The concerns I would have with that is.......would the depth of the hole have to be a specific length to in turn have the spring compressed a specific amount on order for that valve to work properly?.....Could I drill the diameter of the hole precise enough for the new brass seat to press fit in tight?.....and could I keep any drill shavings from dropping into the port?

2) I did buy a disc/drum rebuild kit. NAPA part number UP 501. They have a different kit for drum/drum.

3) I don't have a clue if RPV valves come in different sizes.

I know the installation is easy. I went out last Saturday morning to do it. Getting the brass seat out was a piece of cake. Then I looked at the small size of the port behind the brass seat and said, "Uh oh!"

The pictures below show the rear drum port of the new master cylinder and then the RPV valve, spring, and new brass insert along side of the much smaller brass insert I pulled out of the new master cylinder

Dave

[JohnKY]

I wonder if you could find a rebuilt original Corvette MC, and use your valve in it?

[L78 steve]

You probably realize this but the valve and spring are turned the wrong direction for installation in the pic. above.

[janobyte]

http://www.chevelles.com/forums/16-brakes-suspension-steering/383364-residual-pressure-valves-rpvs-more-i-read-less-i-know.html

Interesting read.

[DAVEN1256]

You probably realize this but the valve and spring are turned the wrong direction for installation in the pic. above.

Yes, I know it's the wrong way. That's just how I set it down to take the picture. After I posted it, I was wondering if someone was going to comment on it.

http://www.chevelles.com/forums/16-brakes-suspension-steering/383364-residual-pressure-valves-rpvs-more-i-read-less-i-know.html

Interesting read.

Yes, it is interesting. Seems this problem has been going on for while now.

Some feel the RPV is not necessary but if GM included them to begin with, the Chassis Service Manual specifically tells you to use them in a drum brake port, and JohnZ says they should be there, that's enough to convince me that I should have one in my master cylinder.

Dave

[TRLAND]

Some feel the RPV is not necessary but if GM included them to begin with, the Chassis Service Manual specifically tells you to use them in a drum brake port, and JohnZ says they should be there, that's enough to convince me that I should have one in my master cylinder.

Agreed on all three points.

[TRLAND]

Although its speculative, the last post in the Chevelle forum post above may be the best explanation I've heard for possibly explaining why some people might get decent braking without RPVs, why vendors don't know about them, and why repro manufacturers have abandoned their use in MC ports.

[janobyte]

Although its speculative, the last post in the Chevelle forum post above may be the best explanation I've heard for possibly explaining why some people might get decent braking without RPVs, why vendors don't know about them, and why repro manufacturers have abandoned their use in MC ports.

Ditto, I've been real curious about this. Across the board, Mustangs,etc, late 60's disc/drum combo cars have faced this issue. Digging for an explanation.

[X33RS]

After all that reading I think too much thought has gone into this, lol.

After pulling my 69Z out of a 30 year slumber I had to replace all the brakes.  Calipers, wheel cylinders, master cylinder.  I can say the brakes are fantastic, stops on a dime with very little pedal movement or pressure.
  I didn't even bother to check for the pressure valve in the master, but it is a correct master bought from HBC.  That doesn't mean it's there, and I'd bet it probably isn't.  But I tend to lean towards the last post in that link posted, that the design of rear wheel cylinders has changed to the point that the pressure valve may not be needed.  Since I changed my rear wheel cylinders I can only guess that is why my brakes are working so well.

I have other disc/drum cars here, that have all had master cylinders and rear wheel cylinders replaced while I've owned them.  Most of them are just store bought rebuilt stuff, and all their brakes also work just fine.  Whether they have pressure valves or not I can't say for sure, but I doubt it since they are equipped with store bought rebuilt $20 master cylinders.  So why are those brakes working so well for me?
   Also just recently, I installed a new master cylinder on my fathers 69GTO, a correct style with bleeder screws (looks just like my 69 Z but without the casting numbers and stamps)  He bought it off Ebay, likely no pressure valve in that one.  I put hydroboost behind it.  Fantastic brakes.  I put wheel cylinders on this car maybe a dozen years ago.   So I'm seeing a pattern here that relates to the last post, so maybe there is something to it.

My thought is, rather than mess around for weeks worrying about pressure valves, then buying rebuild kits for $30 just to get one piece out of it only to find out it doesn't fit, why not grab a pair of rear wheel cylinders for the same money, swap those along with the master cylinder change, and be done with it??  A lot less headache, and less time spent.  If the wheel cylinders are old it's probably time for some preventive maintenance anyway 

[janobyte]

It's the operation of these old systems which is interesting to me.

[DAVEN1256]

First off, I appreciate all of the replies and thoughts on this.

It is going to be a while before this car is actually running on the street so no matter which way I go, I wouldn't know the results for a while.

So now my question is......if rear wheel cylinders were redesigned with cup expanders starting in 1970 eliminating the need for the RPVs.......when you order new wheel cylinders for a 1968 car, what are you getting? The original 1968 design which would mean you still need an RPV......or an updated design meaning you would not?

And if all replacements wheel cylinders were updated, then what happens when you install the updated design (not requiring an RPV) on a car that still has it's original master cylinder that has an RPV. Could that push the brake shoes out too far? Those are just the thoughts running through my head.

This seems to be one of those many subjects where there are two distinct camps. I went through it with silicone vs regular brake fluid and stainless vs regular steel brake lines. In this case we have the folks who have no RPV and have great brakes......and we have the folks who had had poor braking, found out they didn't have an RPV and installed one, and had a great improvement in their braking......It makes it hard to make a good decision on which way to go.

Dave